I answered that you have to uninsatll then reinstall because of the following KB http://support.microsoft.com/kb/914158#top.

It states: To work around the problem (of not being able to upgrade after the evaluation expires), you must uninstall SQL Server 2005 Evaluation Edition. Then, install the retail version of SQL Server 2005.

So the QOD answer for today should be that it depends on if the evaluation edition is expired or not.